An emission type electronic display device includes an electroluminescence display (hereinafter, referred to as an ELD). A constituent element of an ELD includes such as an inorganic electroluminescent element and an organic electroluminescent element (hereinafter, referred to as an organic EL element). An inorganic electroluminescent element has been utilized as a flat light source, however, it requires a high voltage of alternating current to operate an emission element.
On the other hand, an organic electroluminescent element is an element provided with a constitution comprising an emitting layer containing a emitting substance being sandwiched with a cathode and an anode, and an exciton is generated by an electron and a positive hole being injected into the emitting layer to be recombined, resulting emission utilizing light release (fluorescence-phosphorescence) at the time of deactivation of said exciton; the emission is possible at a voltage of approximately a few to a few tens volts, and an organic electroluminescent element is attracting attention with respect to such as superior viewing angle and high visual recognition due to a self-emission type as well as space saving and portability due to a completely solid element of a thin layer type.
Further, the major feature of the organic electroluminescent elements is also in the form of a surface light source differing from conventionally employed main light sources such as a light emitting diode or a cold-cathode tube. Possible applications, which can effectively utilize the above characteristic, include light sources for lighting and backlights of various displays. In particular, it is also appropriate to employ them as a backlight of liquid crystal full color displays, of which demand is markedly increasing over recent years.
When the organic electroluminescent elements are employed as the above light source for lighting or the display backlight, they are employed as a light source which has white or so-called electric bulb color (hereinafter together referred to as white). In order to realize white light emission by employing the organic electroluminescent elements, there available are: a method which realizes white by a color mixture by preparing a plurality of light emitting materials differing in their emitted light wavelength in a single element; a method which realizes white in such a manner that light emitting pixels of multi colors, such as three colors of blue, green, and red, are separately coated, the above pixels are allowed to emit light simultaneously, and then the emitted lights are mixed; and a method which realize white employing color conversion dyes (for example, a combination of a blue light emitting material and a color conversion fluorescent dye).
However, when consideration is made based on various demands such as lower cost, higher productivity, or more convenient driving methods, which are demanded for the light source for lighting and the backlight, the method, which realizes white by a color mixture by preparing a plurality of light emitting dopants differing in emitted light wavelengths in a single element, is useful for these applications, and in recent years, research and development of the same have been increasingly conducted.
The method which realizes white based on the above method will further be detailed. There are listed: a method which realize white in such a manner that two light emitting dopants in a relation of complementary colors to each other, such as a blue light emitting dopant and a yellow light emitting dopant, and the colors are mixed, and a method which realizes white in such a manner that light emitting dopants of three colors of blue, green, and red, are employed and the colors are mixed.
For example, a method for preparing a white organic electroluminescent element by doping with high efficient phosphors of three colors of blue, green and red, as a light emitting material, is disclosed (for example, Patent Documents 1 and 2).
Further, there is a system in which, in organic electroluminescent elements having white light emission, each of the layers differing in emitted light color is not in the form of an individual layer, but light emitting dopants of at least two colors are allowed to coexist in a single layer, and two color lights are allowed to emit via an energy transfer from a light emitting dopant with high light emitting energy to a light emitting dopant with a relatively low efficiency. The above method is one of the promising methods for preparing a white light emitting organic EL element since it is possible to reduce the number of organic layers and to decrease the employed amount of light emitting dopants. For example, in Patent Document 3, an organic electric field light emitting element is disclosed, which is characterized in that a red light emitting layer and a blue light emitting layer are sequentially arranged from the anode, and the red light emitting layer incorporates at least one green color light emitting dopant.
In recent years, there have been actively investigated phosphorescence emitting dopants combined with the phosphorescence materials by which an organic electroluminescent element having a high luminance can be achieved (for example, refer to Patent Document 4, Non-patent Documents 1 and 2). A light emission previous reported utilizes emission from an excited singlet. Since a generation ratio of a singlet exciton to a triplet exciton is 1:3, that is, a generation probability of an emitting exciton species is 25%. On the other hand, in the case of a phosphorescence dopant which utilizes an emission from an excited triplet state, the upper limitation of the internal quantum efficiency will be 100% by considering the production ratio of excitons and internal conversion of a singlet exciton to a triplet exciton. The emission efficiency will be theoretically four times at maximum of the fluorescence emitting dopant.
However, when a white light emitting organic EL element is prepared by using a system containing phosphorescence dopants, in which light emitting dopants of at least two colors are allowed to coexist in a single layer, and two color lights are allowed to emit via an energy transfer from a light emitting dopant with high light emitting energy to a light emitting dopant with a relatively low efficiency, electric power efficiency, life time and color stability against driving electric current are not sufficient. The present situation is that the known methods and conditions for using phosphorescence dopants cannot provide a required effect.                (Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 6-207170        (Patent Document 2) JP-A No. 2004-235168        (Patent Document 3) WO 2004/077886        (Patent Document 4) U.S. Pat. No. 6,097,147        (Non-Patent Document 1) M. A. Baldo et al., Nature, Volume 395, pages 151-154 (1998)        (Non-Patent Document 2) M. A. Baldo et al., Nature, Volume 403, No. 17, pages 750-753 (2000)        